A theory for the nematic-isotropic (N-I) phase transition of prolate uniaxial molecules with longitudinal dipole moments is presented The theory is based on the variational cluster expansion, truncated after the two-molecule term, and is implemented for polar hard spherocylinders with and without attractions, and for polar linear arrays of Lennard-Jones interactions centre We find that the dipole interactions substantially shift the N-I transition temperature and strongly promote antiparallel molecular association, but have a weak effect on the order parameters, the pressure, and the N-I coexistence densities The effect of dipoles on phase stability is very sensitive to their position within the molecular frame Off-centre dipoles are shown to give rise to phase re-entrance according to the sequence N-I-N on heating at constant density The theory does not predict a stable ferroelectric nematic phase

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Electric dipoles and phase stability in nematic liquid crystals

The pretransitional behavior of dodecylcyanobiphenyl (12CB) (isotropic-smectic-A-solid mesomorphism) with d=50nmBaTiO(3) nanoparticles (NPs) linked to the cubic phase was monitored via temperature studies of dielectric constant. Tests were carried out in the isotropic, liquid crystal mesomorphic, and solid phases. For each phase transition the same value of the critical exponent α∼0.5 was obtained, including nanocolloids. All phase transitions show the weakly discontinuous nature. The temperature metric of the discontinuity ΔT notably decreases when adding nanoparticles. The addition of nanoparticles first decreases the dielectric constant by approximately 50% in comparison with pure 12CB, but already for a concentration ∼x=0.4% NP an increase over 50% takes place. It is notable that for the latter concentration unique hallmarks of the pretransitional effect emerge also for the solid-mesophase transition. All these indicate the important impact of nanoparticles on multimolecular mesoscale fluctuations.

Impact of BaTi O 3 nanoparticles on pretransitional effects in liquid crystalline dodecylcyanobiphenyl

The Raman spectra of Terepthalylidene-bis-p-n-declyaniline (TBDA), have been recorded as a function of temperature in the temperature range from 55.0°C (solid) to 199.0°C (isotropic liquid) in the 1100–1200cm−1 and 1520–1650cm−1 regions. The precise values of peak positions, integrated intensities and linewidths of some selected Raman bands have been obtained by deconvoluting the bands. The changes in the molecular alignment and its effect on inter/intra molecular interactions at two particular phase transitions viz., solid-smectic G (K-SG ) and smectic I-Smectic C (SI-Sc ) have been discussed on the basis of variation in the Raman spectral parameters with temperature. From detailed considerations, it is inferred that the increased orientational/vibrational freedom of long alkyl chain on the core is responsible for major spectral changes at the K-SG transition while the changes in the intramolecular interactions of the core part with increased fluidity, is responsible for spectral anomalies at th...

Laser Raman Study of Terepthalylidene-bis-p-n-Decylaniline (TBDA)

Scientists aspire to understand the underlying physics behind the formation of instabilities in soft matter and how to manipulate them for diverse investigations, while engineers aim to design materials that inhibit or impede the nucleation and growth of these instabilities in critical applications. The present paper reviews the field-induced rotational instabilities which may occur in chiral smectic liquid-crystalline layers when subjected to an asymmetric electric field. Such instabilities destroy the so-named bookshelf geometry (in which the smectic layers are normal to the cell surfaces) and have a detrimental effect on all applications of ferroelectric liquid crystals as optical materials. The transformation of the bookshelf geometry into horizontal chevron structures (in which each layer is in a V-shaped structure), and the reorientation dynamics of these chevrons, are discussed in details with respect to the electric field conditions, the material properties and the boundary conditions. Particular attention is given to the polymer-stabilisation of smectic phases as a way to forbid the occurrence of instabilities and the decline of related electro-optical performances. It is also shown which benefit may be gained from layer instabilities to enhance the alignment of the liquid-crystalline geometry in practical devices, such as optical recording by ferroelectric liquid crystals. Finally, the theoretical background of layer instabilities is given and discussed in relation to the experimental data.

/pdf/smectic-layer-instabilities-in-liquid-crystals-1062m7zuxd.pdf

Smectic layer instabilities in liquid crystals

The basic structural differences between three thermotropic liquid crystals TB4A, TB7A and TB10A have been interpreted by analysing their Raman spectra. Three spectral regions, 925-1025, 1275-1475 and 1525-1650 cm-1, have been chosen in this context to incorporate their structural dissimilarity. The induced planarity of the backbone, as an effect of increasing chain length, has been studied by observing the changes in the respective Raman spectra of the three compounds. A tentative assignment of all the modes observed in the region 400-1700cm-1 is made in this context. The molecular conformations of the three compounds have been predicted in their solution states, and are compared with the molecular environment that exists in their liquid crystalline states. Structural disorder at the solid-SmG transition is discussed and the changes are incorporated systematically.

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A comparative laser Raman study on TB4A, TB7A and TB10A

/pdf/thermotropic-liquid-crystals-critical-reports-on-applied-2uy0oqnukh.pdf

Thermotropic liquid crystals. Critical reports on applied chemistry. Vol 22 edited by G. W. Gray and Thermotropic liquid crystals, fundamentals. Springer series in chemical physics. Vol 45 by G. W. Vertogen and W. H. de Jeu

P. G. Barber

Papers

Thermotropic liquid crystals. Critical reports on applied chemistry. Vol 22 edited by G. W. Gray and Thermotropic liquid crystals, fundamentals. Springer series in chemical physics. Vol 45 by G. W. Vertogen and W. H. de Jeu